Low-profile capillary for wire bonding

ABSTRACT

A wire bonding tool includes a first cylindrical portion having a first outside diameter and a second cylindrical portion adjacent the first cylindrical portion. The second cylindrical portion has a second outside diameter, the second outside diameter being less than the first outside diameter. The wire bonding tool also includes a tapered portion adjacent the second cylindrical portion. The tapered portion has a third outside diameter at an end adjacent the second cylindrical portion, the third outside diameter being less than the first outside diameter.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of U.S. patent application Ser. No.11/122,939, filed May 5, 2005 now U.S. Pat. No. 7,320,425, which claimspriority to U.S. Provisional Application No. 60/570,341, filed on May12, 2004, the disclosure of which is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to wire bonding, and more particularly toa capillary used to feed wire in a wire bonding apparatus.

BACKGROUND OF THE INVENTION

In the electronics industry, conductive metal wire is used in a varietyof devices, such as semiconductor devices, to connect contact points onthe device to other contact points. The most commonly used materials forwire bonding are gold and aluminum, although copper and silver are alsoused at times depending on the application. A wire bond is formed byattaching a length of wire between two contact locations. In order toform the attachment, various devices are used to sever and bond (e.g.,melt) the wire ends to the contact location. Some of the most commondevices used to sever and melt the wire are thermocompression (T/C),thermosonic (T/S) or ultrasonic (U/S) devices. The wire is typicallyformed with a generally parabolic or elliptical shape and is, thus,referred to as a wire “loop”.

Two well known techniques for bonding a wire to contact locations of anelectronic device are ball bonding and wedge bonding. Ball bonding isgenerally the preferred technique, particularly in the semiconductorindustry in which more than 90 percent of all semiconductor devices aremanufactured using ball bonding.

Ball bonding apparatuses include a bond head carrying a wire bondingtool such as a capillary. A capillary is an elongated, tubular structureand has an axial passage through which a length of wire is fed forbonding by the bonding apparatus. Ball bonding apparatuses alsotypically include an electronic flame-off (EFO) wand that, when fired,supplies a spark that melts an end portion of the wire extending fromthe capillary. As the molten end portion of the wire solidifies, surfacetension forms the end portion into a substantially spherical shape. Thespherically shaped portion of the wire formed by the EFO wand isreferred to as a “free-air ball”. The free-air ball is bonded to one ofthe contact points on the semiconductor device or substrate by plasticdeformation of the ball onto the contact.

Referring to FIG. 1, there is shown a conventional capillary 10. Thecapillary 10 includes an elongated shaft 12 having a substantiallycylindrical portion 14 and a conical portion 16. As shown, the capillary10 defines an axial passage 18 extending through the capillary forpassage of a wire to be bonded by a wire bonding apparatus. The axialpassage 18 is located substantially concentric with the centerline ofthe capillary 10. The capillary 10 also includes a working tip 20extending from conical portion 16 of the shaft 12 and located at aterminal end of the capillary 10. The working tip 20 of the capillary 10is adapted to form wire bonds through plastic deformation andinterfacial interaction at contact locations, for example, on asubstrate surface. The working tips of known capillaries vary inconfiguration. An example configuration for the working tip of acapillary for a wire bonding apparatus is described in U.S. Pat. No.6,715,658.

As shown in FIG. 1, the conical portion 16 of the capillary shaft 12widens from the working tip 20 at an angle, which is sometimes referredto as the cone angle for the capillary 10. The cylindrical portion 14 ofthe capillary shaft 12 is engaged by a transducer (not shown) adjacent aterminal end 22 of the capillary 10 opposite the working tip 20. Thetransducer vibrates the capillary shaft 12 to supply ultrasonic energyat the working tip 20 of the capillary 10. The ultrasonic energysupplied by the transducer facilitates the above-described plasticdeformation and interfacial interaction between the wire and the contactpoints at a bond site location. As shown in FIG. 1, the diameter ofshaft 12 of the prior art capillary 10 remains substantially constant inthe cylindrical portion 14 from the terminal transducer-engaging end ofthe shaft 12 to the intersection between the cylindrical portion 14 andthe conical portion 16.

It is desirable that the diameter of the free-air ball formed at the endof the wire be aligned as closely as possible with the centerline of acapillary. Concentricity between the free-air ball and the capillarycenterline is desirable for ensuring accurate placement of the wire bondwith respect to a targeted contact location. It would be desirable forthe EFO wand to be located in substantial alignment with the centerlineof the capillary of a wire bonding apparatus. Such an arrangement wouldprovide the greatest probability of concentricity between the resultingfree-air ball formed at the end of the wire by the EFO wand and thecapillary centerline.

Clearance between the capillary and the EFO wand is typically provided,however, to provide access for the working end of the capillary to thecontact locations on a substrate surface. Accordingly, the EFO wandcannot be concentrically aligned with the wire diameter and, instead,must be located at a distance from the centerline of the associatedcapillary. As a result, the spark from the EFO wand is directed to theterminal end of the wire along a path that is oblique with respect tothe capillary centerline.

SUMMARY OF THE INVENTION

According to an exemplary embodiment of the present invention, a wirebonding tool is provided. The wire bonding tool includes a firstcylindrical portion having a first outside diameter and a secondcylindrical portion adjacent the first cylindrical portion. The secondcylindrical portion has a second outside diameter, the second outsidediameter being less than the first outside diameter. The wire bondingtool also includes a tapered portion adjacent the second cylindricalportion. The tapered portion has a third outside diameter at an endadjacent the second cylindrical portion, the third outside diameterbeing less than the first outside diameter.

According to another exemplary embodiment of the present invention, awire bonding system is provided. The wire bonding system includes a wirebonding tool and an EFO wand configured to form a free-air ball at anend of the wire bonding tool. The wire bonding tool includes a firstcylindrical portion having a first outside diameter and a secondcylindrical portion adjacent the first cylindrical portion. The secondcylindrical portion has a second outside diameter, the second outsidediameter being less than the first outside diameter. The wire bondingtool also includes a tapered portion adjacent the second cylindricalportion. The tapered portion has a third outside diameter at an endadjacent the second cylindrical portion, the third outside diameterbeing less than the first outside diameter. The wire bonding system mayinclude various other components (e.g., an ultrasonic transducer, wirespooling mechanisms, a bonding plane, an indexing system, etc.) as isknown to those of ordinary skill in the art.

BRIEF DESCRIPTION OF THE DRAWINGS

For the purpose of illustrating the invention, there is shown in thedrawings a form that is presently preferred; it being understood,however, that this invention is not limited to the precise arrangementsand instrumentalities shown. In the drawings:

FIG. 1 is a side view illustrating a capillary for a wire bondingapparatus according to the prior art;

FIG. 2 is a side view of a capillary for a wire bonding apparatusaccording to an exemplary embodiment of the present invention;

FIG. 3 is graphical illustration with photographic inserts illustratingthe relationship between spark angle and free-air ball concentricity inaccordance with an exemplary embodiment of the present invention;

FIG. 4A is a side view of a portion of a wire bonding system during anindexing operation in accordance with an exemplary embodiment of thepresent invention;

FIG. 4B is a side view of a portion of a wire bonding system during aflame-off operation in accordance with an exemplary embodiment of thepresent invention;

FIG. 4C is a side view of a portion of a wire bonding system during awire bonding operation in accordance with an exemplary embodiment of thepresent invention; and

FIG. 4D is a detailed side view of a capillary and an electronicflame-off wand in accordance with an exemplary embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE DRAWINGS

According to the present invention there is provided a capillary for awire bonding apparatus. The capillary includes an axial passageextending through the capillary for passage of a length of wire. Theaxial passage is preferably substantially aligned with a centerline ofthe capillary. Certain outer dimensions of the capillary are reduced toallow for closer spacing between the centerline of the capillary and thewand of an electronic flame-off device (EFO). Closer spacing between theEFO wand and the centerline of the capillary allows the spark angle tobe increased, thereby resulting in decreased asymmetry between thefree-air ball formed at the end of a wire and the centerline of thecapillary.

In certain exemplary embodiments, the capillary of the present inventioncomprises a conical portion and a substantially cylindrical portionadjacent the conical portion. The cylindrical portion of the capillaryincludes first and second segments, the second segment extending fromthe conical portion. The second segment of the substantially cylindricalportion has an outer diameter that is reduced with respect to that ofthe first segment.

Referring to the drawings, where like numerals identify like elements,there is illustrated in FIG. 2, a capillary 24 according to the presentinvention for use with a wire bonding apparatus. As described in greaterdetail below, the capillary 24 of the present invention allows anelectronic flame-off (EFO) wand to direct a spark towards a terminal endof a wire carried by the capillary 24 at a spark angle that desirablyreduces asymmetry of free-air balls formed by the EFO wand. The reducedfree-air ball asymmetry provided by the present invention results inincreased accuracy in the placement of wire bonds at bond site locationstargeted by the capillary 24.

The capillary 24 of the present invention includes a shaft 26 and aworking tip 28 located at a terminal end of the shaft 26. The particularconfiguration of the working tip 28 is not critical to the presentinvention. A suitable configuration for the working tip 28 of capillary24 is described in U.S. Pat. No. 6,715,658, which is incorporated hereinby reference in its entirety. Similar to the prior art capillary 10, thecapillary 24 includes an axial passage 30 extending in substantialalignment with the centerline of the capillary 24.

The capillary includes a tapered or conical end portion 32, and acylindrical portion 34. The cylindrical portion 34 includes first andsecond segments 36, 38. The first segment 36 has a diameter that isgreater than the diameter of the second segment 38. The diameter andlength L3 of the first segment 36 is sized for engagement with aconventional transducer (not shown). Since the transducer only mounts toa portion of the capillary, only that portion may have the thickened orsturdier capillary wall structure. Preferably, the length L3 of thefirst segment 36 would be between approximately 0.120 inches andapproximately 0.144 inches.

The remainder of the capillary can be reduced in size so as to permitthe EFO wand to be placed closer to the working tip. Accordingly, asshown in FIG. 2, the second segment 38 has a diameter that is less thanthe diameter of the first segment 36 over its entire length L2. Thesecond segment 38 is contiguous with the conical portion 32 and thefirst segment 36. The outer diameter of the shaft 26 in the secondsegment 38 of the cylindrical portion 34 is, preferably, substantiallyconstant throughout the second segment 38. An important factor affectingthe minimum outer diameter for the second segment 38 of the cylindricalportion 34 is wall thickness. As shown in FIG. 2, the diameter of theaxial passage 30 of capillary 24 is variable. The diameter of thepassage 30 decreases in the direction of the working tip 28 tofacilitate insertion of a wire into the axial passage 30 and to guidethe wire towards the open end of the working tip 28. Accordingly, thewall thickness in the cylindrical portion 34 of capillary 24 will be aminimum in the second segment 38 adjacent the juncture with the firstsegment 36 because the outer diameter of the second segment 38 isconstant throughout the length L2. Stresses created in the wall of thecapillary 24 by the ultrasonic vibrations applied by a transducer willbe largest in the relatively thin-walled portion of the second segment38 adjacent the first segment 36. To provide sufficient robustness forthe capillary 24, the minimum wall thickness in the second segment 38 ofthe cylindrical portion 34 is preferably at least approximately 0.003inches.

The conical portion 32 of shaft 26 is adjacent at one end to the workingtip 28 and having an outer surface that widens along a cone angle α.Referring again to FIG. 1 and comparing the conventional capillarydesign to the capillary 24 of the present invention, the cone angle α ofcapillary 24 is preferably the same as or only slightly greater thanthat of the conventional capillary. A preferred cone angle for capillary24 is approximately 20 degrees. The length, L1, of the conical portion32 of the capillary 24 shown in the exemplary embodiment in FIG. 2,however, is significantly less than the length L′ of the conical portion16 of conventional capillaries. As shown in FIG. 1, the length L′ of theconical portion 16 of a conventional capillary 10 is about one-half ofthe overall length of the capillary 10. In the present invention, thelength L1 of the conical portion 32 of the capillary 24 is less. Avariety of factors will determine the optimum ratio between the lengthL1 and the cone angle α including capillary dynamics, the resonantfrequency of the ultrasonic system and bond pad pitch. The ratio of L1/αshould not be reduced to the extent that configuration of the conicalportion 32 of capillary 24 would interfere with adjacently bonded wires.

As a result, the reduced diameter of the second segment 38 and theshorter length of the conical portion 32 not only permit closerplacement of the EFO wand to the working tip 28 compared to that ofconventional capillaries, but also reduce, albeit minimally, the overallmass of capillary.

The reduction in the outer dimensions of the second segment 38 permitsthe EFO wand to be positioned closer to the centerline of the capillary24 without detrimentally affecting the ability of the capillary 24 tomove between a raised position and a lowered position during a bondingoperation. By locating the EFO wand more closely to the centerline ofthe capillary 24 than was previously practical, the spark path angle canbe changed to one more closely approximating an ideal, aligned,configuration. As discussed previously, an aligned configuration wouldresult in the least amount of asymmetry for the free-air ball formed bythe EFO wand at the end of the wire.

As a non-limiting example, assume a transducer (not shown) accepts acapillary having a diameter of 0.0625 inches at the upper end of thecapillary. Accordingly, the first segment 36 of the cylindrical shaftportion 34 of capillary 24 would typically have a diameter of 0.0625inches. The diameter of the second segment 38 of the cylindrical shaftportion 34, however, has a reduced diameter of, for example, 0.0375inches. As a result, the diameter in the second segment 38 is reduced by0.025 inches compared to prior art capillaries which incorporate aconstant diameter of 0.0625 inches throughout the cylindrical portion ofthe shaft. The reduction of 0.025 inches in the diameter of the shaft 26means that the EFO wand can be placed 0.0125 inches closer to thecenterline of the capillary 24.

Assuming that the spark angle is defined as the angle of the spark pathwith respect to horizontal (i.e., a 90 degree spark angle would be avertically-oriented spark path), the spark angle associated with theprior art capillary 10 having 0.0625 inch diameter throughout itscylindrical portion is approximately 40 degrees. The exemplaryembodiment of the present invention described in the precedingparagraph, on the other hand, permits the EFO wand to be placed moreclosely to the centerline of the capillary 24, increasing the sparkangle to approximately 52 degrees.

Referring to FIG. 3, a graphical illustration shows the relationshipbetween spark angle and asymmetry of the resulting free-air balls formedat the end of a wire. The dark circles represent test data points inwhich free-air balls were formed using varying spark angles and theasymmetry between the free-air ball and the wire diameter on which thefree-air ball was formed was measured. A second order curve was thencalculated for the test data points using curve-fitting calculations. Asshown, the resulting second order equation is:y=0.0016x ²−0.3244x+14.876  Eq. 1

Where: x=spark angle (degrees) and y=ball asymmetry (μm).

According to the resulting curve, an increase in spark angle fromapproximately 40 degrees to approximately 52 degrees results in adecrease in free-air ball asymmetry from approximately 4.5 microns toapproximately 2.4 microns. Thus, the closer spacing of the EFO wandprovided by the reduced capillary profile of the present inventionresults in a nearly 50 percent reduction in free-air ball asymmetry.

FIG. 3 also includes inset photographs associated with three of theactual test data points showing the asymmetry between the free-air balland the wire on which the free-air ball is formed. As shown, theasymmetry decreases with increasing spark angle and is nearly eliminatedwhen the spark angle is increased to approximately 65 degrees.

FIG. 4A illustrates wire bonding tool 410 (e.g., capillary 410) in araised position with respect to bonding surface 400. Various componentsare omitted in figures, and the sizes and positions of certain of theillustrated elements are arbitrarily reduced or increased for clarity.Clamping tool 402 is also shown in a raised position, such as when anindexing operation is being performed (e.g., substrates are indexed intoposition on bonding surface 400). Bonding tool 410 includes firstcylindrical portion 410 a, second cylindrical portion 410 b, and conicalportion 410 c. Bonding tool 410 is illustrated as being engaged withtransducer 408 (through first cylindrical portion 410 a). EFO wand 404is also illustrated, and includes EFO tip 406. With clamping tool 402(and bonding tool 410) in the raised position of FIG. 4A, it is clearthat there is a limited location for the position of EFO wand 404including EFO tip 406.

FIG. 4B illustrates clamping tool 402 in a lowered position with respectto bonding surface 400. In this position the “firing” of EFO wand 404may be accomplished to form a free-air-ball at an end portion of a wire(not illustrated in FIG. 4B) extending through bonding tool 410.

FIG. 4C illustrates bonding tool 410 in a lowered position forperforming a wire bonding operation with respect to a device on bondingsurface 400. Bonding tool 410 extends through an aperture defined byclamp 402 (the aperture is not visible in FIG. 4C). During theillustrated wire bonding operation, EFO tip 406 is positioned a distancefrom second cylindrical portion 410 b, wherein the distance is less thanthe difference between the outside diameter of first cylindrical portion410 a and the outside diameter second cylindrical portion 410 b (in theillustrated embodiment the distance is also less than the differencebetween a radius of first cylindrical portion 410 a and a radius ofsecond cylindrical portion 410 b).

As shown in FIG. 4C, it is clear that EFO wand 404 (including EFO tip406) is closer to the centerline of bonding tool 410 than it would be ifsecond cylindrical portion 410 b had the same diameter as firstcylindrical portion 410 a. Thus, during “firing” of the EFO wand (e.g.,in the position shown in FIG. 4B), the EFO tip being closer to thecenterline of bonding tool 410 reduces asymmetry in a formedfree-air-ball.

FIG. 4D is a detailed view of the spark angle in the firing positionillustrated in FIG. 4B. As provided above, the spark angle is the angleof the spark path with respect to horizontal. As is clear from FIG. 4D,the spark angle will desirably increase as EFO wand 404 (including EFOtip 406) is moved closer to the centerline of bonding tool 410. Thus, asprovided above, by providing bonding tool 410 with second cylindricalportion 410 b having a diameter smaller than first cylindrical portion410 a (not shown in FIG. 4D), the spark angle desirably increases.

Although the present invention has been described primarily in terms ofa wire bonding tool defining a tapered passage having a constant taperangle along its length, it is example limited thereto. Alternativeconfigurations of the passage are contemplated, for having, asubstantially linear passage combined with a tapered passage, a taperedpassage having varying taper angles, etc.

The present invention may be embodied in other specific forms withoutdeparting from the spirit or essential attributes thereof and,accordingly, reference should be made to the appended claims, ratherthan to the foregoing specification, as indicating the scope of theinvention.

1. A wire bonding system comprising: a transducer; a wire bonding toolconfigured to be engaged with the transducer; and an EFO wand configuredto form a free-air ball at an end of the wire bonding tool, the wirebonding tool comprising: (a) a first cylindrical portion having a firstoutside diameter, (b) a second cylindrical portion adjacent the firstcylindrical portion, the second cylindrical portion having a secondoutside diameter, the second outside diameter being less than the firstoutside diameter, and (c) a tapered portion adjacent the secondcylindrical portion, the tapered portion having a third outside diameterat an end adjacent the second cylindrical portion, the third outsidediameter being less than the first outside diameter, wherein an end ofthe first cylindrical portion adjacent the second cylindrical portiondoes not extend below the transducer during engagement of the wirebonding tool with the transducer.
 2. The wire bonding system of claim 1wherein the second outside diameter is substantially identical to thethird outside diameter.
 3. The wire bonding system of claim 1 whereinthe wire bonding tool defines a passage extending through each of thefirst cylindrical portion, the second cylindrical portion, and thetapered portion.
 4. The wire bonding system of claim 3 wherein thepassage extends along a centerline of the wire bonding tool.
 5. The wirebonding system of claim 3 wherein the passage is tapered such that adiameter of the passage decreases from the first cylindrical portion tothe tapered portion.
 6. The wire bonding system of claim 1 wherein thetapered portion has a conical shape.
 7. The wire bonding system of claim1 wherein a tip of the EFO wand is configured to be positioned adjacentthe second cylindrical portion during a wire bonding operation.
 8. Thewire bonding system of claim 1 wherein the second outer diameter issubstantially constant along a length of the second cylindrical portion.9. The wire bonding system of claim 1 wherein the first outside diameteris approximately 0.0625 inches, and the second outside diameter isapproximately 0.0375 inches.
 10. The wire bonding system of claim 1wherein, during a wirebonding operation of the wire bonding system, atip of the EFO wand is positioned a distance from the second cylindricalportion, wherein the distance is less than the difference between thefirst outside diameter and the second outside diameter.
 11. The wirebonding system of claim 1 wherein a position of a tip of the EFO wandduring a firing operation of the EFO wand defines a spark angle withrespect to a horizontal plane, the spark angle being greater than 40degrees.
 12. The wire bonding system of claim 1 wherein a tip of the EFOwand is configured to be positioned adjacent a lower end of thetransducer during a wire bonding operation.
 13. The wire bonding systemof claim 1 wherein a tip of the EFO wand is configured to be positionedadjacent (1) the second cylindrical portion and (2) a lower end of thetransducer during a wire bonding operation.
 14. The wire bonding systemof claim 1 wherein a length of the first cylindrical portion is between0.120 inches and 0.144 inches.